In the art of vehicle suspension systems there have been several developments directed to improving the isolation of the vehicle frame from stresses and excursions of the vehicle resulting from uneven or rough roadway conditions and other factors inducing vibration in the vehicle undercarriage. Hydraulic, and/or combination pneumatic and hydraulic suspension systems are desirable for several reasons including improved fatigue life of the mechanism, vibration damping characteristics, reduction of peak force transmission and the contribution of the mechanism to flexibility and the design of vehicle undercarriage and frame. However, prior art hydraulic suspension spring and vibration damper mechanisms have disadvantages which have presented certain problems detracting from their ability to fulfill the suspension needs frequently sought by the owner or operator of the vehicle.
For many years, the suspension system most commonly utilized on roadway type vehicles such as passenger cars has been comprised of the combination of a coil spring and shock absorber at the front wheels and a coil spring or leaf spring and shock absorber at the rear wheels. Operating characteristics of each were typically selected by the automobile manufacturer for the average anticipated loads to be encountered and/or the ride comfort level sought to be maintained. By far, the majority of such systems lack any form of on-site adjustment to accommodate variances between actual load requirements and/or preferred comfort levels other than that originally provided by the manufacturer. Where increased loading was to be encountered subsequently, the suspension components were either replaced with others having increased load capacity or external members were applied to the individual spring units whereby to somehow limit, control or cushion their stroke. Some shock absorber units have included pneumatic adjustment features for loading compensation, leveling or both.
Shock absorbers are generally constructed of three basic designs. The conventional double tube hydraulic shock absorber has an outer reserve tube of hydraulic fluid and air surrounding its inner working chamber. The reserve fluid is used to compensate for changes in volume that occur in the pressure tube as a result of piston rod movement. The single tube gas pressurized or de-carbon shock has a separate pressure chamber containing nitrogen gas at about 400 psi in line beneath the working chamber. The two chambers are separated by a floating piston which moves up and down to balance the pressure of the gas below and the oil above as the volume of the working chamber changes with piston rod travel. The double tube gas shock absorbers have an outer reserve tube like conventional shocks, however, the hydraulic fluid is charged with low pressure gas. When the shock extends, the fluid has a positive pressure preventing cavitation under most circumstances while keeping the damping valving free of foam so as to effect a more consistent damping.
With the advent of the smaller autos many of which have front wheel drives, lesser space than before was available for installing the suspension system and for which the McPherson strut has been largely adopted by the automobile manufacturers. The McPherson strut essentially comprises a combination of the previous functions of the separate spring and shock absorber. At least some of such struts include the feature of damping adjustment operable by opening and closing of orifice means to control the flow rate of the damping fluid during excursions of bounce and rebound. Load capacity has substantially been preset such that the steel spring rate is maintained constant without affording any adjustment capability.
Clearly lacking in such suspension systems of the prior art is the ability to vary the spring rate individually by wheel via an on-site adjustment to more specifically meet the desired or changed suspension requirements to be sustained by the vehicle. Different individuals purchasing the same model vehicle may indeed have different preferences for the response characteristics of the suspension system. While most will be satisfied with the system provided by the manufacturer, at least some will prefer or require substantially different suspension characteristics from that originally provided with the vehicle at the time of manufacture. By way of examples, the elderly frequently prefer a softer more comfortable ride whereas the young may prefer the harder ride for racing or whatever. Buses or tractor-trailers for example may also prefer one characteristic over another depending on the type of cargo, cargo weight to be transported and/or degree of comfort to be afforded. The type of terrain or road conditions may to a large extent dictate or at least influence the desired suspension characteristics. The virtue of having the flexibility and versatility of custom tailoring the properties of the vehicle suspension system have therefore been known but despite recognition thereof an economically satisfactory solution therefor has heretofore been unknown.